Absorption is the process of moving nutrients from the intestines into the bloodstream. Most nutrients are absorbed through the lining of the small intestine, but water-soluble nutrients are absorbed differently from fats and fat-soluble compounds.
Digestion is a series of steps that break down foods into their chemical parts. Foods are composed of starches (carbohydrates), proteins, fats, and a long list of vitamins, minerals, and other things, all stuck together with various mechanical, electrical, and chemical bonds.
Metabolism converts nutrients into forms that can be used (or stored in the body), and also into forms that are removed (excreted) from the body. The technical word for the latter is xenotoxins or just toxins. The use of 'toxin' does not mean that these things are poisons, only that they are not accumulated within the body. The desirable ones have a part to play but are removed when that role is accomplished.
